Summary:
Formate dehydrogenase-H is one of three membrane-associated formate dehydrogenase isoenzymes in E. coli [Sawers94]. All are functional in the anaerobic metabolism of the organism. Formate dehydrogenase-H (FDH-H) is located in the cytoplasm and together with hydrogenase-3, FDH-H forms the formate-hydrogen lyase complex [Axley90, Sawers94].

The enzyme is oxygen sensitive and contains selenium as selenocysteine incorporated cotranslationally at the position of an in-frame UGA stop codon in the FdhF open reading frame [Zinoni86].

A crystal structure of FDH-H has been solved at 2.3 Å resolution, confirming the presence of a [4Fe-4S] cluster, coordination of the Mo cofactor by selenocysteine, and the position of the binding site for the inhibitor nitrate [Boyington97].

Expression of fdhF is induced by formate and the absence of external electron acceptors, and is repressed by nitrate, nitrite, trimethylamine N-oxide, and oxygen [Wu87, Pecher83, Abaibou97]. Formate can overcome repression by nitrate but not by oxygen [Pecher83].

Hydrogenase 3 functions primarily in the production of H2 [Maeda07] and is important for H2 production at acidic pH [Bagramyan02, Mnatsakanyan04, Noguchi10]. Hydrogen uptake in a strain lacking hydrogenase 1 and hydrogenase 2 is further reduced by the incorporation of a hycE mutation, suggesting that hydrogenase 3 can also function in hydrogen uptake [Maeda07]. Hydrogenase 3 shows a high tolerance to product (H2) inhibition [McDowall14].

Hydrogenase 3 is a membrane associated H2 evolving respiratory [NiFe] hydrogenase. It contains the large (HycE) and small (HycG) subunits that are characteristic of 'standard' NiFe hydrogenases plus two additional hydrophilic subunits (HycB and HycF) and two inner membrane subunits (HycC and HycD). Fe-S prosthetic groups located in the hydrophilic part of the complex may form the electron transport pathway (reviews: [Vignais01, Hedderich05, Vignais08]). Isolation of FHL using affinity chromatography indicates the presence of a core complex containing HycE, HycB HycF HycG and FdhF which has formate hydrogenlyase activity in vitro; a larger complex containing the membrane assoicated subunits HyC and HycD is isolated in the presence of detergent [McDowall14]

Sequence similarity between the genes encoding hydrogenase 3 and those encoding subunits that form the core of energy conserving NADH:quinone oxidoreductase (complex I) has been reported [Bohm90] and an evolutionary relationship between the two has been proposed [Hedderich04].

Strains with insertion mutations of genes within the hyc operon are defective for hydrogenase activity [Pecher83, Yerkes84].

The hyc operon is regulated coordinately with the structural gene for formate dehydrogenase H. Expression is repressed by oxygen and by nitrate and induced by formate under fermentative growth conditions [Pecher83, Yerkes84]. Formate is an obligate inducer of the formate hydrogenlyase complex genes [Birkmann87].

Summary:
The hycBCDEFG genes in E.coli K-12 encode the hydrogenase component (hydrogenase 3) of the formate hydrogenlyase complex. hycD encodes an extremely hydrophobic protein with 8 predicted transmembrane domains. The protein sequence has homology with one of the subunits of NADH:ubiquinone oxidoreductase of the respiratory chain [Bohm90, Sawers04].

Summary:
The hycBCDEFG genes in E.coli K-12 encode the hydrogenase component (hydrogenase 3) of the formate hydrogenlyase complex. hycC encodes an extremely hydrophobic protein with 12 to 16 predicted transmembrane domains. The protein sequence has homology with one of the subunits of NADH:ubiquinone oxidoreductase of the respiratory chain [Bohm90, Sawers04].

Summary:
The hycBCDEFG genes in E.coli K-12 encode the hydrogenase component (hydrogenase 3) of the formate hydrogenlyase complex. HycF is thought to be a 4Fe-4S ferredoxin type protein, an intermediate electron carrier protein [Bohm90, Lin91a, Sauter92]. The protein resembles one of the subunits of NADH:ubiquinone oxidoreductase of the respiratory chain [Bohm90, Sawers04].

Summary:
The hycBCDEFG genes in E.coli K-12 encode the hydrogenase component (hydrogenase 3) of the formate hydrogenlyase complex. HycG has similarity to small subunits of hydrogenases [Sauter92] and resembles one of the subunits of NADH:ubiquinone oxidoreductase of the respiratory chain [Bohm90, Sawers04].

Summary:
The hycBCDEFG genes in E.coli K-12 encode the hydrogenase component (hydrogenase 3) of the formate hydrogenlyase complex. HycB is believed to be a peptide of the [4Fe-4S] ferredoxin type, which functions as an intermediate electron carrier protein between hydrogenase 3 and formate dehydrogenase [Lin91a, Sauter92, Trchounian00].

A hycB deletion mutant loses molecular hydrogen production and 2H+/K+ exchange abilities under anaerobic glucose-fermenting conditions. It is suggested that HycB is part of the formate-hydrogen lyase complex that interacts directly with the F0F1 ATPase complex and the TrkA system [Trchounian00].

Summary:
The hycBCDEFG genes in E.coli K-12 encode the hydrogenase component (hydrogenase 3) of the formate hydrogenlyase complex. HycE is the large subunit of hydrogenase 3 [Bohm90, Sauter92]. The protein has homolgy with one of the subunits of NADH:ubiquinone oxidoreductase of the respiratory chain [Bohm90, Sawers04].

Maturation of HycE requires incorporation of nickel followed by processing after the Arg537 residue by the HycI maturation endopeptidase [Rossmann94, Rossmann95, Binder96, Theodoratou00]. Maturation has been reconstituted in vitro and requires HypB, HypC, HypD, HypE, HypF, HycI and nickel, as it does in vivo [Maier96]. HypC interacts directly with pre-HycE and facilitates metal incorporation [Drapal98, Blokesch02]; mutational studies of conserved cysteine residues have led to a model for nickel incorporation [Magalon00]. After the incorporation of nickel, pre-HycE must dissociate from HypC to become a substrate for the HycI maturation endopeptidase [Magalon00a].

Protein engineering of HycE was able to produce a variant with 23-fold higher hydrogen production and nine-fold higher yield on formate compared to wildtype E. coli [Maeda08].